1. Field of the Invention
This invention relates to a switching power supply circuit and, more specifically, to a switching power supply circuit useful when detecting a current flowing in the circuit, and providing feedback on current information to obtain a predetermined output voltage.
2. Description of the Related Art
In a switching power supply circuit, a current feedback mode is in wide use for maintaining the stability of the switching power supply circuit.
This type of switching power supply circuit according to an earlier technology is shown in FIG. 8. As shown in this drawing, the illustrated switching power supply circuit is a step-up switching power supply circuit for performing synchronous rectification. This switching power supply circuit has a switching means SW1 formed from an N-channel MOS transistor, a switching means SW2 formed from a P-channel MOS transistor, and an inductor L, designed to convert a direct current input voltage VIN into a predetermined direct current output voltage VOUT.
An error amplifier 1 outputs an error signal Verr representing a differential voltage between a preset reference voltage VREF and a voltage obtained by dividing the direct current output voltage VOUT by resistances R1 and R2. A PWM comparator 2 compares the error signal Verr with an oscillation signal RAMP outputted by an oscillation circuit 3, and exercises on/off control of the switching means SW1 to SW2 by a switching signal SWS of a predetermined duty determined based on the comparison. More concretely, a buffer circuit 4 forms switching signals SWS1 and SWS2, based on the switching signal SWS, so as to turn off the switching means SW2 during the ON-period of the switching means SW1, thereby turning the switching means SW1 and the switching means SW2 on and off alternately. That is, synchronous rectification is performed. In the drawing, C1 signifies an output capacitor, and C2 signifies a speed-up capacitor.
The above switching power supply circuit provides a current feedback system in order to stabilize control. That is, based on a current ISW flowing into the switching means SW1, a current information signal S11 is formed by utilization of a voltage at the point of connection between the switching means SW1 and a current detection resistance Rsen. The current information signal S11 is supplied to a differential amplifier 6 to obtain a current superposition signal S12, which is added to the aforementioned error signal Verr (substantially, subtracted from the error signal Verr, because the sign of the current superposition signal S12 is minus) at a subtracter 5, whereby a new error signal Verr-r is obtained. This error signal Verr-r is compared with the oscillation signal RAMP by the PWM comparator 2.
The current information signal S11 is supplied to an inverting input terminal of the differential amplifier 6 having a non-inverting input terminal grounded.
FIGS. 9A to 9D are waveform charts showing the waveforms of respective portions of the switching power supply circuit shown in FIG. 8. FIG. 9A shows the waveform of the current ISW, and FIG. 9B shows the waveform of the current information signal S11. FIG. 9C shows the waveforms of the error signal Verr, the new error signal Verr-r (dotted line), which is the output signal of the subtracter 5, and the oscillation signal RAMP, and FIG. 9D shows the waveform of the switching signal SWS which is the output signal of the PWM comparator 2.
As will become clear by reference to FIGS. 9A to 9D (especially, FIG. 9C), in the switching power supply circuit according to the above-mentioned earlier technology, the current superposition signal S12 proportional to the current ISW flowing into the switching means SW1 is subtracted from the error signal Verr. If the current ISW increases or decreases, therefore, it is necessary to adjust the output of the error amplifier 1 in conformity with the action of the PWM comparator 2. As a result, the operating voltage of the error amplifier 1 increases, posing the problem of aggravating transient response characteristics.
Japanese Unexamined Patent Publication No. 2005-33936 can be named as a known patent document which discloses a switching power supply circuit having a current feedback system as mentioned above. The switching power supply circuit disclosed in this patent document samples current information at the turn-on of the switch each time switching takes place. However, this method involves the problem that the circuit is liable to instability, because feedback on a change in current occurring during a load change is impossible.